You need to write code that depends on the JDK release.

Don’t do this.

Although Java is meant to be portable, there are some significant variations in Java runtimes. Sometimes you need to work around a feature that may be missing in older runtimes, but want to use it if it is present. So one of the first things you want to know is how to find out the JDK release corresponding to the Java runtime. This is easily obtained with System.getProperty( ):

System.out.println(System.getProperty("java.specification.version"));

Running this on Java 2 prints “1.2”, as in JDK 1.2. Alas, not everyone is completely honest. Kaffe 1.5 certainly has some features of Java 2, but it is not yet a complete implementation of the Java 2 libraries. Yet it happily reports itself as “1.2” also. Caveat hactor!

Accordingly, you may want to test for the presence or absence of particular classes. One way to do this is with Class.forName("class") , which throws an exception if the class cannot be loaded -- a good indication that it’s not present in the runtime’s library. Here is code for this, from an application wanting to find out whether the JDK 1.1 or later components are available:

/** Test for JDK >= 1.1 */
public class TestJDK11 {
    public static void main(String[] a) {
            // Check for JDK >= 1.1
        try {
            Class.forName("java.lang.reflect.Constructor");
        } catch (ClassNotFoundException e) {
            String failure = 
                "Sorry, but this version of MyApp needs 
" +
                "a Java Runtime based on Java JDK 1.1 or later";
            System.err.println(failure);
            throw new IllegalArgumentException(failure);
        }
        System.out.println("Happy to report that this is JDK1.1");
        // rest of program would go here...
        return;
    }
}

To check if the runtime includes the Swing components with their final names,^[9] you could use:

Class.forName("javax.swing.JButton");

It’s important to distinguish between testing this at compile time and at runtime. In both cases, this code must be compiled on a system that includes the classes you are testing for -- JDK 1.1 and Swing, respectively. These tests are only attempts to help the poor backwaters Java runtime user trying to run your up-to-date application. The goal is to provide this user with a message more meaningful than the simple “class not found” error that the runtime will give. It’s also important to note that this test becomes unreachable if you write it inside any code that depends on the code you are testing for. The check for Swing won’t ever see the light of day on a JDK 1.1 system if you write it in the constructor of a JPanel subclass (think about it). Put the test early in the main flow of your application, before any GUI objects are constructed. Otherwise the code will just sit there wasting space on Java 2 systems and never getting run on Java 1.1 systems.

As for what the class Class actually does, we’ll defer that until Chapter 25.